I think you need new 56t crank...

Just buy the 55T or 56T chainring from BBS and then change it. See if the existing chain can accommodate. If cannot , tukar lagi.

Not enough to climb hill or not enough to go down hill or not enough fast for flat?

Double or triple chainring. Will get you sorted out. What is your current chainring and Cassatte size?
How many gear step up and down you think you lack.

Tern D9 comes with 52T 1x gives 28-93GI (gear inch), so 53T don't make much difference, a realistic 56T @11t will give approx 100GI would be nice, 60 is ideal but don't see them selling very often.
39T @36t will give about 21GI, can climb up a vertical wall liao
56/39 double chainring gives 21-100GI and more higher cadance workout
I assume you consider 30km/h as "fast". If you want >40kmh, get a kapchai.

11-36t is already more than enough to climb any hill..for many, 52-11 is already fast enough on the straight. 30++kmh is totally easily doable on the flat. my guess is, the reason why you felt it is not enough on the straight is due to the bigger jumps in the cassette. and why you felt it is not enough for climbing is probably because of the 52t chainring. yours is using MTB components. the cheaper option is as suggested by other forummers, upgrade to double chainring. IMO, 53-39 is already sufficient. to change the jumps ratio in the cassette, i guess there's no other options but to change to RB cassette, but then, you need different rd, shifters and probably chains as well (if u upgrade it to 10 speed). better change the whole groupset.  

one other option is, which is the cheapest of all and most probably has been said too many times, which is, ride more, and at the same time, before you know it, you'll be also upgrading your legs and lungs 

edit: wait, i just noticed. upgrading to double chainrings also means you need to add fd and shifter as well on your list. 

Ride for another 300km before making any decisions. See if it's you or the bike need any upgrades.
Take your time to dial in your fitting geometry, I.e. seat post height, handlebar position, tire pressure, shifter/brake level position… etc It's very rewarding and it's free.
Average speed can tell a lot while getting through the 1st 500km.

2 possible causes
1) chain too long which is very common as local bike shop like to install longest possible on the safe side
2) rear derailleur need tuning.

I am in the process of making a small leather bag just enough for Mycard/driviing licence and some cash(no coins) and a small space for car keys without the chain ring though and snap under the saddle. Was actually thinking of the road bike market. But mobile phone, a bit susah!

its not the frame la i think... my ka16 also had the same sound, but it came from my handlepost last time i climbed jerai. flat road also small2 climb no problemo...wanna test fraser this sabtuday...

I very slow, has to use low gear to keep the bike moving…

Cadance meter? I use cheap magnetic cyclo computer, place the magnet on the chainring or crank arm, reset the wheel circumference to 16666

And strava on hand phone for speed and distance…
Until I give in to GPS computer sooner or later

Iam using Garmin Edge 810. I can customise its displays to suit my need. U can upload OSM maps no need to buy from Garmin. Can link with Strava too.

16" and 20" conquering Fraser Hills. You dont need top-notch upgrades or hi-end bicycles to tackle this route. Even a mongoose without upgrades can finish ahead of me. (Verge X20)

The most important thing is your guts and motivation. And of course a balls of steel. (Literally cuz i cant feel mine due to numbness at the peak).